Implantable medical devices (IMDs) have many functions including the delivery of therapies to cardiac patients, neuro-stimulators, muscular stimulators, and others. For purposes of this application reference will be made only to implantable cardiac devices, it being understood that the principles herein may have applicability to other implantable medical devices as well.
An implantable cardiac device (ICD) may be a device commonly referred to as a pacemaker, which is used to stimulate the heart into a contraction if the sinus node of the heart is not properly timing, or pacing, the contractions of the heart. Modern cardiac devices also perform many other functions beyond that of pacing. For example, some cardiac devices may also perform therapies such as defibrillation and cardioversion as well as providing several different pacing therapies, depending upon the needs of the user and the physiologic condition of the user's heart. For convenience, all types of implantable cardiac devices will be referred to herein as ICDs, it being understood that the term, unless otherwise indicated, is inclusive of an implantable device capable of administering any of a number of therapies to the heart of the user.
In typical use, an ICD is implanted in a convenient location usually under the skin of the user and in the vicinity of the one or more major arteries or veins. One or more electrical leads connected to the pacemaker are inserted into or on the heart of the user, usually through a convenient vein or artery. The ends of the leads are placed in contact with the walls or surface of one or more chambers of the heart, depending upon the particular therapies deemed appropriate for the user.
One or more of the leads is adapted to carry a current from the pacemaker to the heart tissue to stimulate the heart in one of several ways, again depending upon the particular therapy being delivered. The leads are simultaneously used for sensing the physiologic signals provided by the heart to determine when to deliver a therapeutic pulse to the heart, and the nature of the pulse, e.g., a pacing pulse or a defibrillation shock.
The sensing of the physiologic signal from the heart requires a very sensitive sensing method since the signals sensed are of quite low amplitude. The presence of external, or non-physiologic, electromagnetic interference (EMI), if the field is large enough, can compromise the cardiac sensing function such that the pacemaker may fail to deliver a needed therapy or may deliver an unwanted therapy. Some types of non-physiologic EMI, such as continuous wave at high frequencies, can easily be distinguished from physiologic signals and can thus be ignored or rejected by the pacemaker circuitry. Other forms of non-physiologic EMI, however, are not easily distinguishable from physiologic signals and therefore can block or override the desired physiologic signals.
Many state-of-the-art ICDs are capable of performing either bipolar or unipolar sensing and pacing in either chamber of the heart. Unipolar pacing requires an elongated lead having only one insulated conductor therein and only one generally distal electrode disposed thereon. In most unipolar configurations the protective canister of the ICD is conductive and functions as an electrode in pacing or sensing. For bipolar pacing and/or sensing a lead having two mutually insulated conductors disposed thereon is required. Typically, one electrode is disposed at the distal end of the lead and is referred to as the “tip” electrode, while the second electrode is spaced somewhat back from the distal end of the lead and is referred to as the “ring” electrode. The current path for bipolar pacing extends from the pulse generator in the ICD, along a first of the two lead conductors to the tip electrode, through the cardiac tissue to the ring electrode and back to the ICD along the second of the two conductors.
Most modern ICDs may be programmed to pace and sense in either the bipolar or unipolar mode. This gives the implanting physician considerable flexibility in configuring an ICD system to suit the particular needs of a given patient or user. Additionally, if one of the two leads in a bipolar ICD were to fail for some reason, (e.g., breakage of a conductor due to metal fatigue, an open outer coil, an ineffective ring set screw connection, poor connections, tissue degradation at the electrode site, oxidation, etc.) it would be necessary to reprogram the ICD into unipolar pacing and sensing mode in order for the ICD to continue to perform properly.
In order to detect the failure of a lead in a bipolar ICD unit the impedance of the leads is monitored continuously and, in the event an impedance is detected that is outside a specified range, the ICD is automatically switched to unipolar pacing and sensing until the problem can be rectified. The switch can take several tens of seconds, however, because the impedance measurement must be confirmed by a series of readings before the switch is made. During this time, no pulses are provided, which results in less than optimal therapy. The delay is necessary because a high impedance reading may be caused by electromagnetic interference (EMI). In such cases an out-of-range impedance may be detected when no lead failure has occurred.
Accordingly, it is desirable provide a mechanism and method such that therapy can continue in the event of an out-of-range impedance measurement
Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.